By: Jim Crozier MSAIRAC
The proper amount of suction superheat is a very important factor in ensuring that a refrigeration cycle equipped with a dry expansion (DX) evaporator has a long and trouble-free life. The standard way of measuring suction superheat in air conditioning machines is to take a suction-pressure reading at the compressor’s service port and a suction line temperature reading about 300mm from the pressure measuring point at the compressor. The saturated suction temperature (SST), found from saturation pressure-temperature tables for the refrigerant in use, is compared to the suction line temperature. The difference is the superheat. To protect the compressor from flooding or insufficient cooling, not less than 6K and not more than 8K, suction superheat at the compressor is considered normal when a thermostatic expansion valve is used as the metering device.
Measuring superheat this way works well when the pressure drop in the suction line is known to be within the standard design criterion of the pressure-equivalent of a 1K drop in saturation temperature over the length of the line. When using R22, this translates into a pressure drop of three per square inch (PSI) or 21 kilopascals (kPa). This criterion is aimed for during the design stage, irrespective of the length of the line and the number of bends involved. In long lines, velocity to entrain oil takes precedence at the expense of pressure drop, but does not mean that very large pressure drops occur.
Setting the superheat
In the case of systems with suction lines with considerably larger pressure drops than standard, the only way to set the suction superheat properly is to measure the pressure at the outlet of the evaporator. This may require adding a pressure measuring point into the suction line at the air handler or adding a T-piece into the expansion valve’s equalising line. The suction superheat can then be set between 6K to 8K at the evaporator outlet, rather than at the compressor, and the line pressure loss ignored. With the line properly insulated the temperature rise between the evaporator and the compressor will be negligible.
In Figure 1, the pressure of 68PSIG (469kPag) was measured at sea level. The SST is 40°F (4,4°C) and the line temperature is 11°C. The superheat is 12R (6,6K).
Something I have heard repeated many times over the years is that if the suction pressure at the compressor is less than 450kPa when using R22 the high pressure (HP) must be increased to bring it up. This it is not by any stretch of the imagination to be regarded as a rule, and raising the HP is not the first action to take before carefully analysing the system. Low suction pressure at the compressor can be due to a variety of factors, with excessive suction line pressure loss being one of them. Figure 8 shows some of the factors Danfoss has published.
The SST in the evaporator is usually between 4°C and 6°C, giving an evaporator pressure from 483kPag to 520kPag respectively at Johannesburg’s altitude, but excessive pressure loss due to an undersized or badly designed suction line will badly skew the relationship of 21kPa pressure difference for R22 between the pressure at the compressor and the pressure in the evaporator. Because of the generally held belief of what the compressor suction pressure must be, TEVs are at times adjusted open by service personnel to get the suction pressure at the compressor ‘right’.
Figure 3 shows the sump of a compressor wet with condensation because the TEV was opened to increase the suction pressure at the compressor. In the relatively dry atmosphere of Johannesburg dew on the sump of a compressor is a clear indication of a very cold sump; which is generally caused by liquid flooding back. In general, you can be sure that when the sump is colder than normal hand-temperature it is too cold and that liquid refrigerant in the oil is the likely cause.
|Figure 3. Very low sump temperature is evidenced by condensation. In Johannesburg, because of the low humidity, condensation at the sump is a sure sign of liquid flooding.|
|Figure 4. Oil foam along with a cold sump is a sign of liquid flooding|
When there is foaming in the oil sight-glass and the sump is colder than hand temperature there is definitely liquid flooding back in significant quantities.
With liquid refrigerant diluting the oil there is a high probability of compressor damage occurring.
Causes of excessive pressure drop in this installation
In this installation the separation distance between the condensing unit and the air handling unit was very long to start with and there were a lot more bends in the run than was absolutely necessary. As bends cause a lot of pressure drop the run must be carefully designed to keep the number used to the absolute minimum. A calculation from having measured the installed line revealed that the suction line pressure drop was significantly greater than the 21kPa design standard for R22.
In addition, suction line dryers had been added to protect the compressor at start up from swarf and dirt in the system. A lot of bends had been used for that too; once again, more than were absolutely necessary. All in all, the pressure drop in the line was so great that the pressure at the compressor was around 380kPag when it was running.
This low pressure was taken by the serviceman on site to be a clear indication that the machine needed more refrigerant, even though the liquid line sight glass was clear. When more refrigerant failed to produce the “required” suction pressure the TEV was adjusted open.
|Figure 5. Suction dryers were fitted. These and all the bends are highly restrictive.|
Firstly, the suction dryers and the extra, associated bends were removed.
Secondly, a pressure measuring point was fitted into the suction line at the AHU. This facilitated setting the suction superheat correctly as per Figure 1.
Thirdly, the over-charge of refrigerant was removed to achieve the correct condensing pressure.
The suction pressure at the compressor was not used to determine the superheat.
Dryers in a reverse cycle machine
This is a reverse cycle or heat pump machine. For those not entirely familiar with such a machine’s operation, a word about positioning dryers and filters in reverse cycle machines might be useful. The refrigerant flow on these machines reverses when the operating mode changes and the suction line becomes the hot gas line. Although the liquid line remains as the liquid line the direction of flow reverses too. Trapped particulate will be forced out of dryers and filters in the liquid and suction lines on a flow reversal unless special dryers with built-in check valves are used.
The dryer shown in Figure 6 (left - Picture from Sporlan)is a liquid dryer, and although the manufacturer indicates that it can be used on the suction line in small machines – up to 3,5kW of cooling – its primary purpose is for liquid.
A dryer , with or without check valves, in the ‘suction’ line of a heat pump remains a problem because when the mode changes and the suction line becomes the hot gas line the temperature rises and moisture that was trapped in the core during the cooling cycle will be driven out; back into the refrigerant. Check valves only keep particulate matter from escaping.
If a suction dryer must be fitted to the system it has to go between the reversing valve and the compressor where the flow direction and temperature remains the same in both modes of operation. This is shown in Figure 7. (right - picture from Trane)
A suction dryer must only be installed in the suction line between the compressor and the four-way valve where suction vapour always flows in the same direction through the dryer, irrespective of the mode of operation of the machine.
The compressor discharge is always into the single pipe on the four-way valve and the suction is always from the centre pipe of the three. Depending on the manufacturer of the air conditioner, the other two pipes could be connected to provide indoor heating if the solenoid coil either has power or has no power. There is no standard in this.
Installing a suction dryer will usually incur having to make changes to the piping.
Refrigerant vapour line sizing for air conditioners.
Pipe length is normally stated as “linear length” and “equivalent length”. The term “total pipe length” is used by some manufacturers when stating the allowable separation distance between the indoor and outdoor parts of the machine.
Linear length is the physical length that can be measured with a ruler. Equivalent length includes the pressure losses from bends and any fittings such as valves and dryers. Using tables, these pressure losses are converted to length and added to the linear length to produce the “equivalent length” or “total pipe length”. The equivalent length is the length that the pipe diameter is selected on.
The diameters of refrigerant lines carrying vapour – hot gas and suction – must be sized sufficiently large to prevent pressure losses from significantly reducing the capacity of the compressor, but not large enough to cause gas velocities to be too low to ensure adequate oil entrainment for return to the compressor. For systems with capacity control this is especially important as the oil must still be entrained at minimum capacity. The accepted norm is to size the pipes carrying vapour so that the pressure losses due to friction are limited to that equivalent to a 1K change in the saturated temperature of the refrigerant in use.
In the case of R22 when being used at air conditioning temperatures, the loss in pressure in suction lines must be limited to 20kPa, and for discharge or hot gas lines the pressure loss must be limited to 40kPa. In both cases the pressure drops in the respective lines equate to a 1K drop in saturation temperature.
It can happen at times that the pressure drop limits have to be exceeded in order to ensure adequate gas velocity to ensure oil entrainment. Oil entrainment always takes precedence.
Saturation tables must be consulted for the refrigerant in use to find the pressure drop equivalent to a 1K drop in saturation temperature.
In the next issue Jim discusses Air Volumes